1. Field of the Invention
The present invention relates to a magneto-optical disc apparatus, particularly to a mechanism for controlling a distance between a magneto-optical disc and a magnetic head.
2. Description of Related Art
Generally, a magneto-optical disc apparatus is so constructed as to record information by projecting light beam to a magneto-optical disc (hereinafter to be called a disc) rotating in order to record information, then by switching polarity of magnetic field to be given to the projected position of the light beam. When the disc is rotated, there is a fear that information may be recorded inaccurately because a distance between the disc and a magnetic head which applies magnetic field to the disc from the exterior is changed due to a warp of the disc thereby to change magnetic field intensity to be given to the disc. Therefore, servo operation of an actuator supporting the magnetic head is performed so that the magnetic head may follow surface deflection of the disc and the distance between the disc and the magnetic head may be constant.
FIG. 1 is a schematic side view of essential parts of such a conventional magneto-optical disc apparatus. A disc 1 rotates together with a rotary shaft 2 rotated by a spindle motor (not shown). Immediately under the disk 1, a distance sensor 4 and a magnetic head 5 are fixed on an actuator 3. The distance sensor 4 and magnetic head 5 are movable in the direction of an arrow of solid line according to exciting current of an actuating coil 3a operating the actuator 3 thereby to control the distance between the disc 1 and the magnetic head 5. The distance sensor 4 detects the distance between the distance sensor 4 and the disc 1 successively be continuously projecting light to the disc 1 and receiving the reflected light as shown in an arrow of broken line, and the exciting current of the actuating coil 3a is controlled corresponding to the detected output of the distance sensor 4.
Next, explanation will be given on the operation of the magneto-optical disc apparatus.
When the distance servo 4 detects the distance between itself and the disc 1, the exciting current flowing in the actuating coil 3a is changed according to the detected output. The actuator 3 is moved in the direction of the arrow of solid line according to the exciting current of the actuating coil 3a thereby to change a distance d between the magnetic head 5 and the disc 1. In this way, the magnetic head 5 follows surface deflection due to the rotation of the disc 1, and the distance d between the disc 1 and the magnetic head 5 is always kept, for example, 500-600 .mu.m, thereby magnetic field intensity to be given to the disc 1 from the magnetic head 5 is made constant.
By the way, as shown in FIG. 2 being a schematic plan view of the disc 1, a plurality of sector marks 1a, 1a, . . . are formed radially from the center of the disc 1, and data area 1b are formed between the adjacent sector marks 1a, 1a. Because each header part is prepitted on each sector mark 1a, there is a possibility that light reflectance is more largely changed at the sector mark 1a than that at the data area 1b. FIG. 3 is a view showing an example of change of detected output of the distance sensor 4, axis of abscissa showing circumferential position of the disc 1 and axis of ordinate showing the detected output of the distance sensor 4. When the distance sensor 4 receives reflected light from the sector mark 1a, spike noise SP whose magnitude is largely changed is generated in the detected output of the distance sensor 4, as shown in FIG. 3, the change being same as that in the state where the distance between the distance sensor 4 and the disc 1 is suddenly changed. Thereby exciting current of the actuating coil 3a of the actuator 3 is to increase largely, leading a problem the servo electric power i to be consumed unnecessarily. And every time the distance sensor 4 receives reflected light from the sector mark 1a, the actuator 3 is actuated, leading a problem that servo operation of the magnetic head 5 is unstabilized. Moreover, at a position of the sector mark 1a, the magnetic head 5 is moved in close to the disc 1 more than necessity, leading a fear that the magnetic head 5 and the disc 1 may be damaged.